davinci: PSC: Clear bits in MDCTL reg before setting new bits

Clear any set bits in the 'NEXT' field of the MDCTL register in the
Power and Sleep Controller (PSC) before setting any new bits.
This also allows some minor cleanup by removing some no longer
needed lines of code.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>